1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to transmitting physical downlink control channels.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from transmission points such as Base Stations (BSs) (or NodeBs) to User Equipments (UEs), and an UpLink (UL) that conveys transmission signals from UEs to reception points such as NodeBs. A UE, which is also commonly referred to as a terminal or a Mobile Station (MS), may be fixed or mobile and may be a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as an Access Point (AP) or some other equivalent terminology.
DL signals include data signals, which carry information content, control signals, and Reference Signals (RSs), which are also known as pilot signals. A NodeB transmits data signals through respective Physical Downlink Shared CHannels (PDSCHs) and transmits control signals through respective Physical Downlink Control CHannels (PDCCHs). UL signals also include data signals, control signals, and RSs. UEs transmit data signals through respective Physical Uplink Shared CHannels (PUSCHs) and transmit control signals through respective Physical Uplink Control CHannels (PUCCHs). It is possible for a UE having transmission of data information to also transmit control information through the PUSCH.
Downlink Control Information (DCI), which serves several purposes, is conveyed through DCI formats transmitted in respective PDCCHs. For example, DCI includes DL Scheduling Assignments (SAs) for respective PDSCH receptions by UEs and UL SAs for respective PUSCH transmissions from UEs. As PDCCHs are a major part of the total DL signaling overhead, their resource requirements directly affect DL throughput.
Accordingly, one method for reducing PDCCH overhead is to scale its size according to the resources required to transmit the DCI formats during a DL Transmission Time Interval (TTI).
Assuming Orthogonal Frequency Division Multiplexing (OFDM) as the DL transmission method, a Control Format Indicator (CFI) parameter transmitted through a Physical Control Format Indicator CHannel (PCFICH) can be used to indicate a number of OFDM symbols for a DL control region in a DL TTI.
A frequency resource unit, which is also referred to as a Resource Block (RB), includes a number of sub-carriers or Resource Elements (REs), e.g., 12 REs. A PDSCH transmission can occur over a number of RBs and over a number of OFDM symbols, e.g., over all OFDM symbols in a DL TTI, after a DL control region.
FIG. 1 illustrates a conventional structure for PDCCH transmissions in a DL TTI.
Referring to FIG. 1, a DL TTI includes a subframe having N=14 OFDM symbols. A DL control region occupies a first M OFDM symbols 110. A remaining N−M OFDM symbols are used primarily for PDSCH transmissions 120, and a PCFICH 130 is transmitted in some REs of a first OFDM symbol and provides 2 CFI bits indicating a DL control region size of M=1, M=2, or M=3 OFDM symbols.
Some OFDM symbols also include RS REs 140 and 150, which assumes that there are two NodeB antenna ports. These RS REs 140 and 150 are transmitted over substantially an entire DL operating BandWidth (BW) and are referred to as Common RSs (CRSs), as they can be used by each UE for performing channel measurements and for demodulating control or data signals. A CRS is transmitted without precoding, i.e., without applying a phase to its transmission, or with a same precoding (phase) as for all control or data signals for which a UE uses the CRS for demodulation.
In addition to a CRS, other RS types for a DL subframe include a DeModulation RS (DMRS) that is transmitted only in RBs used for PDSCH transmission to a UE (the DMRS uses UE-specific precoding and is associated with a data signal using a same precoding), and a Channel State Information (CSI) RS that is periodically transmitted in some subframes, without UE-specific precoding, and is intended to primarily serve for measurement.
Additional control channels may be transmitted in a DL control region but are not shown for brevity. For example, when using a Hybrid Automatic Repeat reQuest (HARQ) process for data transmission in a PUSCH, a NodeB may transmit a Physical Hybrid-HARQ Indicator CHannel (PHICH) to indicate to a UE whether a previous transmission of data Transport Blocks (TBs) in a PUSCH was correctly received, i.e., an ACKnowledgement (ACK), or incorrectly received, i.e., a Negative ACK (HACK).
Each Control CHannel (CCH) transmitted with the conventional structure illustrated in FIG. 1 will be referred to herein as a conventional CCH (cCCH) including a cPDCCH, a cPCFICH, and a cPHICH.
FIG. 2 illustrates a conventional encoding process for a DCI format.
Referring to FIG. 2, a NodeB separately codes and transmits each DCI format in a respective cPDCCH. A Radio Network Temporary Identifier (RNTI) for a UE for which a DCI format is intended for masks a Cyclic Redundancy Check (CRC) of a DCI format codeword in order to enable a UE to identify that a particular DCI format is intended for it. Alternatively, a DCI-type RNTI may mask a CRC, if a DCI format provides UE-common information. The CRC 220 of (non-coded) DCI format bits 210 is computed and it is subsequently masked 230 using an eXclusive OR (XOR) operation between CRC and RNTI bits 240. For example, the XOR operation is XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0, and both a CRC and an RNTI have 16 bits.
The masked CRC bits are appended to DCI format information bits 250, followed by channel coding 260, e.g., using a convolutional code, by rate matching 270 to allocated resources, interleaving and modulating 280, and transmitting a control signal 290.
FIG. 3 illustrates a conventional decoding process for a DCI format.
Referring to FIG. 3, a received control signal 310 is demodulated and de-interleaved 320, a rate matching applied in a NodeB transmitter is restored 330, and data is subsequently decoded 340. After decoding, DCI format information bits 360 are obtained after extracting CRC bits 350, which are then de-masked 370 by applying an XOR operation with a UE RNTI 380 (or a DCI-type RNTI). Finally, a UE performs a CRC test 390.
If the CRC test passes, a UE considers a DCI format as valid and determines parameters for signal reception or signal transmission. If the CRC test does not pass, a UE disregards a presumed DCI format.
To avoid a cPDCCH transmission to a UE blocking a cPDCCH transmission to another UE, a location of each cPDCCH transmission in the time-frequency domain of a DL control region is not unique. Consequently, a UE must perform multiple decoding operations to determine whether there are cPDCCHs intended for it in a DL subframe.
The REs carrying each cPDCCH are grouped into conventional Control Channel Elements (cCCEs) in the logical domain. For a given number of DCI format bits, a number of cCCEs for a respective cPDCCH depends on a channel coding rate, e.g., Quadrature Phase Shift Keying (QPSK)).
A NodeB may use a lower channel coding rate and more cCCEs for a cPDCCH transmission to a UE experiencing a lower DL Signal-to-Interference and Noise Ratio (SINR) than to a UE experiencing a higher DL SINR. The possible cCCE aggregation levels for a cPDCCH transmission may be, for example, 1, 2, 4, and 8 cCCEs.
For a cPDCCH detection process, a UE may determine a search space for candidate cPDCCH transmissions after it restores cCCEs in the logical domain according to a common set of cCCEs for all UEs (i.e., a Common Search Space (CSS)) and according to a UE-dedicated set of cCCEs (i.e., a UE-Dedicated Search Space (UE-DSS)). For example, a CSS includes the first C cCCEs in the logical domain, and a UE-DSS may be determined according to a pseudo-random function having as inputs UE-common parameters, such as the subframe number or the total number of cCCEs in a DL subframe, and UE-specific parameters such as a RNTI. For example, for cCCE aggregation levels Lε{1,2,4,8}, cCCEs corresponding to a cPDCCH candidate in are given in Equation (1).cCCEs for cPDCCH candidate m=L·{(Yk+m)mod └NCCE,k/L┘}+i  (1)
In Equation (1), NCCE,k is a total number of cCCEs in subframe k, i=0, . . . , L−1, m=0, . . . , MC(L)−1, and MC(L) is a number of cPDCCH candidates to monitor in a search space. Exemplary values of MC(L) for Lε{1,2,4,8} are, respectively, {6, 6, 2, 2}. For a CSS, Yk=0. For a UE-DSS, Yk=(A·Yk 1)mod D, where Y−1=RNTI≠0, A=39827 and D=65537.
If enough cCCEs remain after transmissions of DCI formats conveying UE-common information, a CSS may also convey some DCI formats for DL SAs or UL SAs. A UE-DSS exclusively conveys DCI formats for DL SAs or UL SAs. For example, a CSS may include 16 cCCEs and support 2 DCI formats with L=8 cCCEs, or 4 DCI formats with L=4 cCCEs, or 1 DCI format with L=8 cCCEs and 2 DCI formats with L=4 cCCEs. The cCCEs for a CSS are placed first in the logical domain (prior to interleaving).
FIG. 4 illustrates a conventional transmission process for cPDCCHs.
Referring to FIG. 4, after channel coding and rate matching, as illustrated in FIG. 2, the encoded DCI format bits are mapped in the logical domain to cCCEs of a cPDCCH. The first 4 cCCEs (L=4), cCCE1 401, cCCE2 402, cCCE3 403, and cCCE4 404 are used for cPDCCH transmission to UE1. The next 2 cCCEs (L=2), cCCE5 411 and cCCE6 412, are used for cPDCCH transmission to UE2. The next 2 cCCEs (L=2), cCCE7 421 and cCCE8 422, are used for cPDCCH transmission to UE3, and the last cCCE (L=1), cCCE9 431, is used for cPDCCH transmission to UE4.
DCI format bits are scrambled 440 by a binary scrambling code and are subsequently modulated 450. Each cCCE is divided into conventional Resource Element Groups (cREGs). For example, a cCCE having 36 REs can be divided into 9 cREGs, each having 4 REs.
Interleaving 460 is applied among cREGs (blocks of 4 QPSK symbols). For example, a block interleaver may be used where interleaving is performed on symbol-quadruplets (4 QPSK symbols corresponding to the 4 REs of a cREG) instead of on individual bits.
After interleaving the cREGs, a resulting series of QPSK symbols is shifted by J symbols 470, and each QPSK symbol is mapped to an RE 480 in a DL control region.
Therefore, in addition to a CRS from NodeB transmitter antenna ports 491 and 492, and other control channels such as a PCFICH 493 and a PHICH (not shown), REs in a DL control region include QPSK symbols for cPDCCHs corresponding to DCI formats for UE1 494, UE2 495, UE3 496, and UE4 497.
FIG. 5 illustrates a conventional transmission process for a CFI.
Referring to FIG. 5, a NodeB transmitter generates CFI bits 510, e.g., 2 CFI bits, encodes the CFI bits and performs a number of repetitions 520 to obtain a sequence of encoded CFI bits. For example, a (3, 2) Hamming code and 11 repetitions of the encoded CFI bits may apply to obtain sequences of 32 encoded bits, after puncturing the last repeated encoded bit. The sequences of encoded bits are modulated using QPSK 530 and the output is mapped to frequency disperse cREGs 540 and transmitted in a cPCFICH 550.
FIG. 6 illustrates a conventional reception process for a CFI.
Referring to FIG. 6, a UE receiver receives a cPCFICH 610, accumulates repeated transmissions of encoded CFI bits over respective cREGs 620, demodulates an accumulated output 630, decodes resulting bits 640, and obtains an estimate of the transmitted CFI bits 650.
The cPHICH cREGs is placed only in a first OFDM symbol or over a maximum of three OFDM symbols of a DL control region. A cPHICH transmission in each cREG is not confined in only one RE, but in order to provide interference randomization, it is spread over all REs in each cREG. To avoid reducing a cPHICH multiplexing capacity (by a factor of 4 for a cREG of 4 REs), orthogonal multiplexing of cPHICH transmissions may apply within each cREG using orthogonal codes with a Spreading Factor (SF) equal to NSF,freqcPHICH. For a cREG of 4 REs, the orthogonal codes are Walsh-Hadamard (WH) codes with NSF,freqcPHICH. For QPSK modulation and 1-bit HARQ-ACK for each data TB received by a NodeB, each cPHICH may be placed on the In-phase (I) or the Quadrature (Q) component of a QPSK constellation and may be modulated with a WH code over each cREG.
For NSF,freqcPHICH, the 1-bit HARQ-ACK multiplexing capacity of each cPHICH is 2NSF,freqcPHICH=8 (obtained from the 2 dimensions of QPSK (I/Q and from NSF,freqcPHICH=4). Therefore, multiple cPHICHs separated by I/Q multiplexing and by different WH codes are mapped to a same set of REs in one or more cREGs and constitute a cPHICH group.
FIG. 7 illustrates a conventional transmission of a HARQ-ACK bit in a cPHICH.
Referring to FIG. 7, a HARQ-ACK bit 710 is multiplied 722, 724, 726, and 728, by each element of the WH code 732, 734, 736, 738 and a resulting output is placed on the I-branch of a QPSK modulated RE 742, 744, 746, and 748 (the Q-branch may be used to transmit another HARQ-ACK bit). The WH code may be one of 4 WH codes 750. With I/Q multiplexing and orthogonal sequence multiplexing with NSF,freqcPHICH=4, 8 cPHICHs are provided within one cREG. As for a CFI transmission, a transmission in each cPHICH group may be repeated over multiple cREGs to obtain frequency diversity and improve the effective SINR of each HARQ-ACK signal.
A UE receiver for a cPHICH only performs the conventional functions of QPSK demodulation and WH code despreading (and averaging over repeated cPHICH group transmissions as discussed below), which is similar to the cPFCICH UE receiver functions illustrated in FIG. 6 (with the exception of WH code dispreading).
A cPHICH resource is identified by a pair (ncPHICHgroup, ncPHICHseq), where ncPHICHgroup is a cPHICH group number and ncPHICHseq is a WH code index within a cPHICH group. A number of cPHICH groups is ncPHICHgroup=┌Ng(NRBDL/8)┐, where Ngε{⅙,½,1,2} is a parameter informed to UEs through a broadcast channel, NRBDL is a total number of RBs in a DL BW, and ┌ ┐ is a “ceiling” operation rounding a number to its next integer.
A UE is normally informed of NRBDL prior to a cPHICH reception, but this may not be possible for the total number of UL RBs, NRBUL, in an UL BW. Accordingly, NRBDL (not NRBUL) is used to specify NcPHICHgroup. A cPHICH group number is determined as shown in Equation (2).ncPHICHgroup=(IRB—RAlowest—index+CSI)mod NcPHICHgroup  (2)
Further, a WH code index within a group is determined as shown in Equation (3).ncPHICHseq=(└IRB—RAlowest—index/NcPHICHgroup┘+CSI)mod 2NSF,freqcPHICH  (3)
In Equations (2) and (3) IRB—RAlowest—index represents a smallest RB index of a PUSCH transmitting a data TB corresponding to a HARQ-ACK bit transmitted in a cPHICH, CSI represents a CSI of a Zadoff-Chu (ZC) sequence used for DMRS transmission in a PUSCH, and └ ┘ is a “floor” operation rounding a number to its previous integer.
cPHICH resources corresponding to consecutive UL RBs are mapped to different cPHICH groups. For simplicity, one data TB per PUSCH is assumed herein, but the above expressions can be generalized for multiple data TBs per PUSCH.
A DL control region for transmitting cCCHs uses a maximum of M=3 OFDM symbols and each cCCH is transmitted over substantially an entire DL BW. Consequently, the DL control has limited capacity and cannot achieve interference co-ordination in the frequency domain.
There are several cases where expanded capacity or interference co-ordination in the frequency domain is used for transmitting CCHs. For example, one such case is a communication system with cell aggregation, where DL SAs or UL SAs for UEs in multiple cells are transmitted in a single cell. Another case is an extensive use of spatial multiplexing, where multiple DL SAs or UL SAs schedule respective PDSCHs or PUSCHs in same respective resources. Another case is when DL transmissions in one cell experience strong interference from DL transmissions in another cell and DL interference co-ordination in the frequency domain between the two cells is needed.
However, due to the cREG-based transmission and interleaving of the cCCHs, a conventional DL control region cannot be expanded to include more OFDM symbols while maintaining compatible operation with existing UEs, which will not be aware of such expansion.
An alternative is to extend a DL control region in the PDSCH region and use individual RBs over a number of subframe symbols for transmitting enhanced CCHs (eCCHs) and include ePDCCH, ePCFICH, and ePHICH.
FIG. 8 illustrates a conventional structure for eCCH transmissions.
Referring to FIG. 8, although eCCH transmissions start immediately after cCCH transmissions 810 and are over all remaining subframe symbols, alternatively, they may start at a fixed location, such as the fourth OFDM symbol, and extend over a part of the remaining subframe symbols. In FIG. 8, eCCH transmissions occur in four RBs, 820, 830, 840, and 850, while the remaining RBs are used for PDSCH transmissions 860, 862, 864, 866, and 868.
However, several aspects for an eCCH operation in FIG. 8 still need to be defined in order to provide a functional design.
One aspect is the RS used for demodulation of eCCHs. A CRS transmitted over an entire DL BW may not always exist or may not always be received by all UEs without significant interference. Therefore, the CRS may not be generally relied upon for demodulating eCCHs. A DMRS is associated with UE-specific precoding and although UE-specific DMRS can be associated with an eCCH transmission to a UE, the UE-specific DMRS cannot be associated with multiple eCCH transmissions to respectively multiple UEs.
Another aspect is the multiplexing and transmission of an ePCFICH and the functionality for a respective eCFI indicating a size of an enhanced control region in a respective DL subframe.
Another aspect is the multiplexing and transmission structure for an ePHICH in an enhanced control region.
Another aspect is the multiplexing and support for a CSS and for a UE-DSS for ePDCCH transmissions and detections.
Therefore, there is a need to define an operation for an enhanced DL control region.
There is another need to define an RS used by multiple UEs for demodulating respective eCCHs transmitted over multiple RBs.
There is another need to define a multiplexing and transmission structure for an ePCFICH and an operation for a respective eCFI.
There is another need to define a multiplexing and transmission structure for ePHICHs.
Finally, there is another need to define a multiplexing and transmission process for an ePDCCH in an enhanced CSS or an enhanced UE-DSS and a UE operation for detecting this ePDCCH.